Use the content width to calculate the zoom overview

width. We used to use the minimum preferred width
to calculate the overview width as it is what we
use to define the viewport width. But some sites,
like cbsnews, engadget, the content width will be
slightly wider than the viewport width decided by
the min preferred width. The result is we can still
scroll a little bit in the overview mode. This issue
is magnified when we introduce the overscroll feature.

By using content width as zoom overview width, we
will have content fit in the overview mode. But
some sites, like
"http://www.sfjazz.org/concerts/2010/spring/artists/towner.php"
will be layout wider in the overview mode than it
is in the zoom in state. This is a tradeoff.

Fix http://b/issue?id=2551119